The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moiré spots reflect graphene in a single orientation epitaxial incommensurate phase.

We present two-dimensional broadband quantum cascade laser arrays based on distributed-feedback (DFB) ring cavity surface emitting lasers. The 16-element arrays exhibit a linear tuning range of centered at a wavelength of when operated in pulsed mode at room temperature. The devices show single-mode emission with a side mode suppression ratio of 30 dB. Given by the facetless nature of the single emitters, the spectral dependent threshold current densities and optical power reflect the gain profile of the incorporated material and are not impaired by the diversity of underlying DFB designs.

Lasing mechanisms within paintable dye-doped chiralliquid crystalemulsions are investigated. Evidence shows that by variation in liquid crystaldroplet size, by simple control of mechanical mixing speeds, a change in the lasing mechanism from band-edge lasing (large droplets) to diffuse nonresonant random lasing (small droplets) can be facilitated. This approach represents a facile technique for the variation in lasing mechanism, within a self-organizing, flexible, and conformable system, and offers the opportunity of developing controllable linewidth laser sources.

We demonstrate reversible and irreversible changes in the ultrafast optical response of multilayergraphene oxide thin films upon electrical and optical stimulus. The reversible effects are due to electrochemical modification of graphene oxide, which allows tuning of the optical response by externally applied bias. Increasing the degree of reduction in graphene oxide causes excited stateabsorption to gradually switch to saturable absorption for shorter probe wavelengths. Spectral and temporal properties as well as the sign of the ultrafast response can be tuned either by changing the applied bias or exposing to high intensity femtosecond pulses.

We report on the fabrication of aluminumgallium nitride(AlGaN)Schottky diodes for extreme ultraviolet(EUV) detection. AlGaN layers were grown on silicon wafers by molecular beam epitaxy with the conventional and inverted Schottky structure, where the undoped, active layer was grown before or after the n-doped layer, respectively. Different current mechanisms were observed in the two structures. The inverted Schottky diode was designed for the optimized backside sensitivity in the hybrid imagers. A cut-off wavelength of 280 nm was observed with three orders of magnitude intrinsic rejection ratio of the visible radiation. Furthermore, the inverted structure was characterized using a EUV source based on helium discharge and an open electrode design was used to improve the sensitivity. The characteristic He I and He II emission lines were observed at the wavelengths of 58.4 nm and 30.4 nm, respectively, proving the feasibility of using the inverted layer stack for EUV detection.

The influences of defects and surface roughness on the indirect bandgap radiative transition of Ge were studied. Bulk Ge has 15 times the integrated intensity of photoluminescence of Ge-on-Si. However, for Ge-on-Si sample, the direct transition related photoluminescence intensity is higher than the indirect transition related one. We affirm that the defects in the Ge-on-Si are responsible for the weak indirect transition and relatively strong direct transition. The scattering of electrons by roughness at Ge/oxide interface can provide extra momentum of the indirect band transition of Ge, and thus enhance the indirect radiative transition.

Whispering gallery mode(WGM)optical resonators utilizing resonance shift (RS) and mode splitting (MS) techniques have emerged as highly sensitive platforms for label-free detection of nanoscale objects. RS method has been demonstrated in various resonators in air and liquid. MS in microsphere resonators has not been achieved in aqueous environment up to date, despite its demonstration in microtoroid resonators. Here, we demonstrate scatterer-induced MS of WGMs in microsphere resonators in water. We determine the size range of particles that induces MS in a microsphere in water as a function of resonator mode volume and quality factor. The results are confirmed by the experimental observations.

A compact configuration for real-time achromatic measurements of space-variant light polarization is presented. The experimental results reveal that the full state of polarization at each location within a light beam or at each wavelength can be obtained with accuracy of over .

We report microspectroscopy measurements of crystalline channel waveguides fabricated in and using the ultrafast laser inscription technique. From these measurements we find that densification of the bridge in the double tungstate crystal lattice is responsible for the refractive index increase, which creates the waveguide confinement. We identified that shifts toward lower energies in the Raman mode indicate regions, which guide light polarized along the crystallographic axis, while higher energy shifts in the 682 and Raman lines correspond to guiding regions for light polarized along the crystallographic axis.

We demonstrate that an ultrashort-pulse laser-driven x-raydiode can be used for time-resolved experiments on a picosecond timescale. Hence, acoustical phonons in germanium are observed after ultrashort laser-excitation and the results are compared with calculations according to a microphysical model. We also show the advantages of this kind of picosecond x-ray source compared to other sources on the basis of its properties.

A scheme for creation of periodic plasma structures by ablating a lithographic pattern is demonstrated. A proof of principle experiment was conducted, and plasma parameters were measured as a function of time with spatial resolution and periodicity. Several possible applications, in particular, quasiphase matching for high harmonic generation in plasma are considered.

We experimentally demonstrate the feasibility of controlling the fluorescence emission of nitrogen molecules in air induced by femtosecond laserfilamentation by using a pump-probe method. An obvious enhancement or reduction in the filament-induced fluorescence signals of nitrogen molecules can be realized when a blue (400 nm) or an infrared (1338 nm) laser pulse is used as the probe. The completely opposite effect is ascribed to the excitation enhancement of ionization and population trapping of some highly excited states including Rydberg states of nitrogen molecules.

We describe the growth and characterization of InAsquantum dots on patternedGaAs by metal organic chemical vapor deposition. Arrays of quantum dots with densities as high as fabricated by electron beamlithography are demonstrated. A process consisting of dry and wet etching to minimize etch damage is developed. As the mask diameter increases, the nucleation transitions from single dots to multidot clusters. We achieve more uniform size and shape distributions of dots on patterned regions relative to unpatterned dots as revealed by structural characterization and room temperature photoluminescence emission spectra.

Results of modeling of streamer propagation along helium jets for both positive and negative polarities of applied voltage are presented. Obtained patterns of streamer dynamics and structure in these two cases are similar to those observed in experiments with plasma jets.

Temporal evolution of the gas temperature in a pulsed microwave surfaguide discharge is studied by measuring the rotational temperature. We found that at high power applied per pulse, gas temperature grows linearly, and saturates after about . This effect is absent at low power values, or at short pulse durations. Observed behavior correlates with time-resolvedmeasurements of the vibrational temperature, as well as with N emission lines. Consequently, time behavior was related to N atoms production in plasma. Using obtained growth rates, the effective power used for plasma heating is determined.

Direct thrust measurements of a permanent magnet helicon double layer thruster have been made using a pendulum thrust balance and a high sensitivity laser displacement sensor. At the low pressures used (0.08 Pa) an ion beam is detected downstream of the thruster exit, and a maximum thrust force of about 3 mN is measured for argon with an rf input power of about 700 W. The measured thrust is proportional to the upstream plasma density and is in good agreement with the theoretical thrust based on the maximum upstream electron pressure.

Interface models between various group IV and III-V (100)semiconductors and their oxides are generated from first-principles molecular dynamics simulations. The tensile strain at the interface (from the semiconductor side) is estimated, by comparing bond lengths at/near the interface and in the bulk semiconductor phase. A linear relationship between the calculated interface stress and the density of interface defects observed at such interfaces is revealed. These results suggest that the interface stress due to the volume mismatch between the semiconductor and its oxide likely plays an important role in the creation of interface defects. These findings can explain recent results pertaining to the passivation of various high-mobility channels for their integration in high-performance metal-oxide-semiconductor field-effect transistors.

Magnetic-field-induced reorientation in Ni–Mn–Ga five-layered martensite (10 M) mediated by the motion of single twin boundary was evaluated from magnetization measurements between 20 and 300 K. At 300 K, the single twin boundary moved in an exceptionally small field of 25 kA/m. Twinning stress, as a measure of the twin boundary mobility, was determined from the magnetization curves using a magnetic-energy-based model; it increased from ≈0.1 MPa at 300 K to ≈0.8 MPa at 20 K. The dependence is discussed in terms of thermal activation and the effect of intermartensitic transformation is considered.

The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moiré spots reflect graphene in a single orientation epitaxial incommensurate phase.

We studied the dephasing dynamics of coherent phonons in n-type, p-type, and intrinsic Si using time-resolvedreflectivitymeasurements with sub-10 fs laser pulses. The dephasing time of the coherent phonons increases (decreases) for n-type (p-type) doping compared with that of intrinsic Si, while the frequencies of the coherent phonons exhibit a redshift for both types of doping. These doping-induced changes in the coherent phonon dynamics are observed when the carrier concentration exceeds . The doping-type dependent changes in the dephasing time are attributed to the interconduction and intervalence band transitions in n-type and p-type Si, respectively.

In this letter, we have investigated the structural properties of thick InGaN layers grown on GaN by plasma-assisted molecular beam epitaxy, using two growth rates of 1.0 and 3.6 Å/s. A highly regular superlattice(SL) structure is found to be spontaneously formed in the filmgrown at 3.6 Å/s but not in the filmgrown at 1.0 Å/s. The faster grownfilm also exhibits superior structural quality, which could be due to the surface roughness suppression caused by kinetic limitation, and the inhibition of the Frank–Read dislocation generation mechanism within the spontaneously formed SL structure.